Previous studies have shown that a toxic metabolite of acetaminophen is formed during the metabolism of the drug in the liver of cytochrome P-450-dependent mixed function oxidase. Evidence has been presented that this toxic reactive metabolite is N-acetyl-4- benzoimidoquinone which arises through N-hydroxylation of the parent drug followed by nonenzymatic rearrangement. Recently we found that phenacetin also causes centrilobular hepatic necrosis in hamsters. Since N-hydroxyphenacetin will spontaneously rearrange to N-acetyl-4- benzoimidoquinone, a known arylating agent, we examined the capacity of hepatic microsomes to N-hydroxylate phenacetin.